Telecommunications systems, cable television systems and data communication networks use optical networks to rapidly convey large amounts of information between remote points. In an optical network, information is conveyed in the form of optical signals through optical fibers. Optical fibers are thin strands of glass capable of transmitting the signals over long distances with very low loss.
Optical networks often employ wavelength division multiplexing (WDM) to increase transmission capacity. In a WDM network, a number of optical channels are carried in each fiber at disparate wavelengths. Network capacity is increased as a multiple of the number of wavelengths, or channels, in each fiber.
Each optical channel carries a data signal and a corresponding clock signal. The clock signal is recovered and used to make forward error corrections (FEC) decisions for the data signal. FEC typically requires good clock extraction which is difficult with very poor waveforms and/or a large amount of jitter present in conventional intensity shift keying systems. To improve clock recovery, a pure clock signal has been transmitted to each node as a data signal. This however is not cost-effective and is complicated from a network operation standpoint.